Yes it's harmful.
It is lead sulphate.
It is toxic to ingest and breath.
It is especially toxic to children, as you would imagine regular lead to be.
It's not miscible in water but baking soda (sodium bicarbonate) neutralizes that acidic portion of the compound. The remnant compound of that reaction would be H2O and some CO.
Use gloves. Lead salts are quite poisonous. You should also wear an air filtering mask and eye protection when cleaning it.
You can put baking soda in some water and mix it. Pour it on the battery terminal and use a brush to clean it off. The water and baking soda will prevent you from breathing in any of the lead sulfate dust. You will see a reaction and it will foam, that will be mostly water and CO with ions of a few other chemicals like lead and sulpher, there is a type of salt that breaks off too.
If you see any brown in the foam that is probably iron oxide getting broken down from an older component of the cable connector, if you have an older vehicle. Modern clamps avoid the use of ferrous metals due to oxidation (rust)
Use distilled water to pour on the foamy mess to wash it off. Ensure you DO NOT get any of the remnants from the reaction on the paint of the vehicle. It will damage it. Ensure you wash the remnants completely out of the vehicle, battery tray, devices below the tray like suspension etc.
Hope this helps. Cheers.